In an optical communication technology, in order for input optical power of an optical receiver to fall within an appropriate reception range, the input optical power may be controlled by a variable optical attenuator (VOA). For example, the input optical power is monitored by using a photodetector (or photodiode) (PD), and an attenuation amount of the VOA is controlled based on the monitoring result.
Related techniques are disclosed in, for example, Japanese National Publication of International Patent Application No. 2013-528982 and Japanese Laid-open Patent Publication No. 2014-039156.